A battery energy storage system (BESS) refers to a system used to store electricity on a large scale within an electrical power grid. For example, when production of electrical power systems exceeds consumption, the BESS is configured to store electrical energy. In this way, electrical energy production does not have to be drastically scaled up and down to meet momentary consumption. Rather, production can be maintained at a more constant level. Thus, electrical power systems can be more efficiently and easily operated at constant production levels.
The BESS can be a stand-alone system or may be incorporated into a renewable energy power system to provide energy storage capability. A stand-alone BESS is generally connected to a power grid via a switching power supply, such as a DC to AC power converter. Further, a typical stand-alone BESS includes one or more energy storage devices (e.g. batteries) coupled to the switching power supply via a DC link and a bridge circuit. Stand-alone BESSs can be used for peak shaving for commercial/industrial plants, buffering peak loads in distribution grids, energy trading, buffering solar power (e.g. at night time), upgrades for solar/wind power generation, and many other suitable applications.
Renewable energy power systems, such as wind energy power systems and solar energy power systems, also typically include a power converter with a regulated DC link much like a stand-alone BESS. For example, wind power systems, such as wind driven doubly-fed induction generator (DFIG) systems or full power conversion systems, typically include a power converter with an AC-DC-AC topology. Solar power systems typically include a power converter that has a DC-DC-AC topology or a single stage DC-AC topology. A BESS may be coupled to the DC link of the power converter of the renewable energy power system and can be used, for instance, to apply power to the DC link of the power converter during transient conditions or to obtain power from the DC link. A switching power supply can be provided to transfer energy back and forth between the DC link of the power converter and the BESS. For instance, the switching power supply is often a DC to DC converter configured to convert a first voltage on the DC link to a second voltage at the BESS, and vice versa. Further, it can be desirable for the switching power supply of the renewable energy power system to be bi-directional to allow not only for power flow from the BESS to the DC link during transient conditions but also to allow power flow from the DC link to the BESS, for instance, to charge the BESS.
In order to supply power to a power grid, the renewable energy power system and/or the energy storage system need to conform to certain requirements. For example, the renewable energy power system and/or the energy storage system may need to offer high-voltage ride through (HVRT) capability or low-voltage ride through (LVRT) capability. HVRT capability refers to the ability of a system to stay connected to the power grid during an over-voltage condition. Similarly, LVRT capability refers to the ability of a system to stay connected to the power grid during a low-voltage condition. Since either of these conditions in the power grid may cause issues with operation of a power converter, however, it is desirable to design energy systems so as to accommodate the high or low voltage events without damaging system components.
For example, various power systems have employed a dynamic brake having a brake chopper and a resistive element configured with the DC link capacitor to regulate the DC link voltage as described in U.S. Pat. No. 8,432,055 entitled “Wind Turbine Having a High-Voltage Ride Through (HVRT) Mode”, which is incorporated herein by reference. In another example, a relatively large and bulky DC link capacitor having a relatively high breakdown voltage may be provided. In still a further example, if a power convertor employs semiconductor switches, such as insulated gate bipolar transistors (IGBTs) or integrated gate commutated thyristors (IGCTs), an over-voltage or over-current condition may exert stress or render the switches in the power converter inoperable. Accordingly, a line inductor that connects the power converter to the power grid may be provided having a relatively high voltage rating to protect the switches located in the power converter. However, providing large, bulky capacitors and inductors adds cost and complexity to the wind turbine.
In view of the foregoing, it would also be advantageous to provide improved HVRT or LVRT capability to energy storage systems and/or renewable energy power systems. Thus, the present disclosure is directed to a system and method for protecting a power converter of a stand-alone energy storage system and/or a power converter of a renewable energy power system during such an adverse voltage event.